Is it zapping anything else? If not, then I guess the +5V line is probably not the cause of the problem, which in turn, means the PSU regulation loop is basically OK (these supplies regulate the 5V line and hope all the others follow :-)). An open-circuit in the sense circuit can make the output go sky-high, but then all outputs tend to go high together. Defective output capacitors in SMPSUs can cause high-ish voltage spikes on the outputs -- and generally only on one output. Perhaps the capacitors for the +12V rail are failing. If you have an ESR meter you could check them. The +12V rail is generally used by the hard drive and RS232 drivers only in modern machines -- the floppy drive probably only needs +5V. And the RS232 drives (if 'traditional') will stand serious overvoltage on the supply lines. -tony